Conventionally, a vehicle navigation system uses both absolute positioning data obtained by a satellite navigation technique using a GPS sensor and relative positioning data obtained by a dead reckoning navigation technique using a gyro sensor, a vehicle speed sensor and the like, to detect an absolute position of a vehicle. The vehicle navigation system displays the current position of the vehicle on a road map indicated based on map data. However, due to, for example, an error between a road on the road map indicated based on the map data and an actual road, the detected absolute position often does not conform to the road map indicated based on the map data.
To avoid this, the processing of correcting the position of the vehicle to match it with a road map indicated based on the map data (map matching process) is performed to find the vehicle position relative to the road map indicated based on the map data. Thus, in the navigation system emphasis is placed on matching a vehicle position with a road map. Therefore, even in a place where a road map indicated based on the map data differs from an actual road, the navigation system is required to display the vehicle position matched with the road map.
On the other hand, the navigation system for vehicle control according to road conditions is used more often recently. For example, US 2005/0027419A1 (JP 2005-59839A) discloses a control system, which adjusts the direction of the optical axes of a pair of right and left headlights mounted on the front end of a vehicle in a horizontal plane with respect to the surface of a road. Thus, the optical axes of the headlights are adjusted in direction according to road conditions, which are determined from a road map indicated based on map data (in a road direction in front of the vehicle). In this regard, the adjustment of the optical axis direction of the headlights according to the road conditions, which are determined based on the road map, involves an inappropriate control relative to the actual road conditions in a place where the accuracy of the road map indicated based on the map data is low, such as mountain paths. For this reason, the optical axis of only one of the pair of right and left headlights is adjusted in direction according to the road conditions, which are determined from the road map indicated based on the map data.
With such a technique of controlling the direction of the optical axis of only one of the headlights in accordance with the road conditions determined from the road map indicated based on the map data, the control deterioration occurring in a place where the accuracy of the road map indicated based on the map data is low (the control results deviate from the actual road conditions) can be reduced by half. However, the control effects in a place where a high accuracy of the road map indicated based on the map data is obtained are also reduced by half.